Machines or devices used at many production sites are typically controlled by control systems including controllers, such as programmable logic controllers (PLCs). Each PLC in the control system includes a central processing unit (CPU) module and functional units such as an input-output (IO) unit that receives an input signal from an external switch or a sensor and/or outputs a signal to an external relay or an actuator. The PLC may be connected to one or more remote IO terminals via a network. Each of the remote IO terminals includes a communication coupler and one or more functional units.
The PLC may communicate with one or more remote IO terminals using polling in which the PLC functions as a master for managing the entire communication. For example, Japanese Unexamined Patent Application Publication No. 2007-312043 (Patent Literature 1) describes two typical modes of communication between a master and slaves in a remote IO system, namely, broadcasting and polling. Broadcasting is a communication mode in which the master station simultaneously transmits signals to all the slave stations. Polling is a communication mode in which the master station obtains identification information of target slave stations, such as their addresses, and transmits signals to the target slave stations by designating the target slave stations.